In aircraft breathing systems where oxygen enrichment may be achieved through on-board oxygen generation, one means of generating an oxygen rich product gas is by fractionalizing air. Air fractionalizing is usually accomplished by alternating the flow of high pressure air through each of two beds of molecular sieve material such as a zeolite. This process is identified as the pressure swing adsorption technique.
The oxygen concentration of the enriched product gas of a pressure swing adsorption system decreases as the product gas flow through the system increases. It is therefore possible to control the oxygen partial pressure (PPO.sub.2) within a fixed range or vary the PPO.sub.2 in accordance with other parameters such as aircraft cabin pressure or altitude and/or breathing system pressure. For example, it could be desirable in a high altitude aircraft to maintain a preset minimum oxygen concentration at lower altitudes and thereafter increase the oxygen concentration as the altitude of the aircraft increases in order to hold the PPO.sub.2 constant.